Time step continuity error unit
 

Hello
I am trying to figure out the unit of the global time step continuity error. Is it in percent, in decimal percent, or something else entirely?

I hope someone can help :-)

No one knows?

Anybody knows?

Hi Thomas

It's a scalar field representing the sum of all the deviations in the continuity equation (grad.u=0) over all cells. Therefore it is representing flux, not percentages!

Take a look a the continuityErrs.H file in
and also this post:
http://www.cfd-online.com/Forums/ope...tml#post201896

Hi, thanks for replying :-)
Ok, so flux, but in what unit? I saw that in the post linked it was for incompressible flow, I'm running rhoSimplecFoam, which is for compressible flows, so the the continuity error file looks like this instead:

{
dimensionedScalar totalMass = fvc::domainIntegrate(rho);

scalar sumLocalContErr =
(fvc::domainIntegrate(mag(rho - thermo.rho()))/totalMass).value();

scalar globalContErr =
(fvc::domainIntegrate(rho - thermo.rho())/totalMass).value();

cumulativeContErr += globalContErr;

Info<< "time step continuity errors : sum local = " << sumLocalContErr
<< ", global = " << globalContErr
<< ", cumulative = " << cumulativeContErr
<< endl;
}

If using SI units, the difference between rho and thermo.rho must be in kg/m^3, right? And the total mass is a volume integral over rho, so that must be in kg? I am unsure what value value() takes, but according to the thread it's usually 1, but in what unit?

So that means the unit of globalContErr is 1/m^3 is that correct? How do I then normalise this so I can get it in percentage?

I don't work with compressible flows myself, but after running an incompressible simulation I can go to any one of the time directories in the case folder and look at the phi file. Near the top I find: dimensions [0 3 -1 0 0 0 0]; which tells me the units of phi (and therefore units of the time step continuity error in my case) is m3/s (i.e. volume flux).

So you can double check your rho files for the units but it looks to me like your units are rho/mass = (kg/m3)/(kg) = m-3.

Hey decah, thanks for replying.

Ok, from that it seems like my flux is mass flow with units kg/s. Do I need to compute the mass flux at the inlets in order to normalise the continuity errors?

I'm not sure what you mean by 'normalise' the errors, but if you want to compute mass flux at inlet you can run

Okay, so if the continuity error is a deviation between what comes into the system and what comes out, then by calculating the flux of the inlet, I should be able to find the global error in percentage by the following formula:

globalContErr/flux_inlet*100

?